Documentation for time-stamping functionality

This commit adds documentation for newly introduced helper commands,
configuration variables and the certificate storage file `.git_tsa_store`. The
documentation pages for `git tag` and git `verify-tag` are altered to describe
the new command line options for time-stamping.

Signed-off-by: Anton Würfel <anton.wuerfel@fau.de>
Signed-off-by: Phillip Raffeck <phillip.raffeck@fau.de>